The invention relates to a method of assembling a semiconductor device package and in particular, a method of assembly which minimises or prevents delamination at interfaces within the assembled package.
Plastic surface mount semiconductor device packages are susceptible to cracking during solder reflow and this phenomenon is commonly referred to as “popcorn cracking”. Popcorn cracking occurs because epoxy molding compounds used to encapsulate semiconductor devices are hygroscopic and readily absorb moisture from the environment. During solder reflow, high hydrothermal stresses are induced due to the combination of rapid vaporization of the absorbed moisture and mismatches in the coefficients of thermal expansion between dissimilar materials in the package. Once the stress level reaches a critical threshold, delamination occurs, usually at the weakest interface, followed by the build-up of vapour pressure in the delaminated cavity forming a characteristic dome-shaped bulge. This leads finally to cracking of the encapsulation material.
Popcorn cracking is a potential reliability problem as the delaminated areas and/or cracks can induce corrosion failures, alter the thermal performance of power devices, and affect the stress distribution and concentration.
The molding compound/die-pad interface is known to be susceptible to delamination and the source of most popcorn failure modes. The situation is aggravated with the emergence of larger and thinner packages. Moreover, moisture can degrade the polymer/metal interfacial durability, and decrease the fracture toughness of the molding compound at reflow temperatures. Furthermore, the oxidation of copper-based leadframe materials during the assembly process can result in poor adhesion between the molding compound and die-pad. The main cause of poor adhesion has been attributed to the weak copper oxide layer on the leadframe surface. Studies have reported that the adhesion strength between the molding compound and the copper leadframe decreases with increasing oxide thickness.
Various techniques to enhance molding compound/die-pad adhesion have been suggested as possible solutions to prevent popcorn cracking. However, most of the solutions which have been proposed do not eliminate the problem of popcorn cracking completely and are either not economically viable and/or feasible to implement in mass production.
For example, organic adhesion promoters such as silane coupling agents are widely used in die-attach adhesives and molding compounds to improve adhesion at the various interfaces. However, they are temperature sensitive and susceptible to degradation at elevated temperatures (typically greater than 200° C.), for example, during wire bonding.
Use of an inorganic zinc-chromium (Zn—Cr) leadframe coating (commercially known as “Olin A2” and supplied by Olin Metal Research Laboratories) is believed to be effective in eliminating popcorn cracking. However, the Zn—Cr coating is deposited on the leadframe electrolytically via an electroplating process. However, due to the temperature stability of the Zn—Cr coating layer, it can prevent or interfere with subsequent solid-state bonding, fusion or soldering processes such as wire bonding, etc. Therefore, it is necessary to either mask the bonding/soldering areas during the coating process or to subsequently strip the coating from these areas. Therefore, there are surfaces of the leadframe and semiconductor device in the finished package which do not have the Zn—Cr coating and are still prone to popcorn cracking.
Chip buffer coatings such as polyimides, are commonly used to minimise thermo-mechanical stresses and delamination at the mold compound/chip interface. However, polyimides have the disadvantage that they have a tendency to absorb moisture and therefore, further contribute to popcorn cracking.